In so-called pressure diffusers which in the cellulose industry are used for washing pulp in continuously working washing columns, the screen unit is reciprocated with the aid of a hydraulic working cylinder via a pull rod. The stroke length is normally up to approximately one meter. During the downward working stroke, the screen unit is fed slowly downwards at a speed which only slightly exceeds the sinking speed of the cellulose pulp in the column. On the other hand, the speed during the return stroke is high, normally approximately 1-1.5 m/s. The screen unit which is to be lifted during this rapid return movement can weigh more than ten tonnes and even larger installations have been planned. Furthermore, friction arises between the screen unit and the cellulose pulp in the column. The working cylinder and other parts in the hydraulic unit must therefore be of large dimensions in order to be capable of performing considerable work for a short time. This involves for example flow rates of the hydraulic oil of more than 10,000 l/min, oil quantities which furthermore have to filtered and cooled during the working cycle of the hydraulic unit. The systems which exist at present do not solve these problems in a satisfactory manner, which is hampering development within this field of technology towards increasingly large pressure diffusers and with this increasingly heavy screen units.
In order to meet the above requirements, the last fifteen years have seen the development of first the so-called float accumulator and then a hydraulic unit which comprises a so-called piston receiver. An advantage of the float accumulator is that it does not have dynamic gaskets which have to be replaced as a result of wear. In the case of greater oil quantities and cycle time, however, the gas losses become so great that gas refilling must take place frequently. The development of the piston receiver therefore constituted an improvement in this field. An example of a unit of this type is illustrated in SE 464533. Instead of gas charging, this unit has a hydraulic cylinder which presses the oil back to the tank. However, the principle has the disadvantage that the piston receiver must be completely, or at least essentially completely empty, before the next rapid return stroke is started. The emptying time must therefore be shorter than the shortest cycle time of the diffuser. This means that the hydraulic oil flow is highly discontinuous with a varying flow quantity. Apart from the fact that this involves difficulties in damping pressure peaks, the flow variation and the discontinuous manner of working also mean that filtering and cooling cannot take place optimally.